FIG. 1 shows a prior art, pseudo-2-phase, charge-coupled device (CCD) shift register. The CCD is typically fabricated as an n-type buried channel 330 at the surface in a p-type well or substrate 360. There are channel potential control gate electrodes 310, 320, 312, and 322 that are used to effect transfer of charge in the CCD. Gates 310 and 320 are electrically connected together to one voltage and gates 312 and 322 are electrically connected together to a second more negative voltage. The implant 350 under gates 320 and 322 modifies the channel potential to control the direction of charge transfer. If the gate 312 is physically very long, then electrons in the CCD feel a very small electric field pushing them forward towards the next gates 320 and 310. The small electric field leads to poor charge transfer efficiency and slow transfer times.
FIG. 2 shows a prior art solution to the small electric fields of FIG. 1. It also has an n-type buried channel 230 in a p-type well or substrate 260. There are channel potential control gate electrodes 270, 275, 280, and 285 that are used to effect transfer of charge in the CCD. Gates 270 and 275 are electrically connected together to one voltage and gates 280 and 285 are electrically connected together to a second more negative voltage. Additional implants 210 and 240 are used to create a step in the channel potential under gates 270 and 280. The extra step in the channel potential increases the electric field for improved charge transfer efficiency.
FIGS. 3 and 4 show the prior art fabrication process of FIG. 2. FIG. 3 shows a point in the fabrication process where only the first level of polysilicon gate 275 and 285 have been formed above the n-type buried channel 230. A blanket n-type implant (indicated by the vertical arrows) is used to increase the channel potential under where the second level polysilicon gate is to be placed. The first level gates 275 and 285 act as a self-aligned mask to block the implant. That implant forms the first step in the channel potential 240. There is also a photoresist mask level at this step in FIG. 3 to block the implant from the circuitry around the ends of the CCD, that mask is not shown in FIG. 3.
Next, in FIG. 4, a photoresist mask 290 is used to partially block an implant from penetrating the area where the second level polysilicon gate is to be placed. This forms the second channel potential step 210 in FIG. 2.
Prior art patents describing CCDs of similar processes shown in FIGS. 3 and 4 are U.S. Pat. Nos. 4,910,569; 5,315,137; 5,379,064; and 6,818,483.
The disadvantage of the fabrication process of FIGS. 3 and 4 is it requires two mask levels. It also requires the channel potential steps to be placed under the second level of polysilicon. Sometimes it is desired for the long gate to be of the first level of polysilicon, not the second.
Consequently, the present invention provides a manufacturing method to create a stepped channel potential under a long first level polysilicon gate using only one implant mask.